Nobukazu Nakasato

Neuromagnetic localization of N15, the initial cortical response to lip stimulus.

The initial cortical response of the trigeminal somatosensory evoked magnetic fields was measured for electrical stimulation of the lower lip in eleven subjects. The stimulus frequency was 0.7 Hz and stimulus intensity was nine times sensory threshold. The initial contralateral response was detected in 20 hemispheres at a latency of 14.6 ± 1.3 ms and was named N15m. The equivalent current dipole of N15m was localized at the posterior bank of the central sulcus with anterior–superior orientation, and inferior to the dipole of N20m for median nerve stimulation.

Functional localization of bilateral auditory cortices using an MRI-linked whole head magnetoencephalography (MEG) system

In 20 healthy subjects, auditory evoked magnetic fields were measured over the entire head, using a helmet-shaped 66-channel MEG system linked to MRI. When the left or right ear was stimulated by 60 msec 2 kHz tones, the prominent 100 msec response (N100m) appeared significantly earlier in the contralateral hemisphere than in the ipsilateral one. In 16 cases, the N100m dipolar field patterns were clear in both hemispheres, overlapping each other across the midline. The N100m sources were estimated using a 2-dipole model in a spherical conducting medium with the size and location of the sphere determined individually according to the MRI images. No differences were found between the contralateral and ipsilateral N100m dipole positions in one hemisphere. When superimposed on MRI, the N100m dipoles were located precisely on the upper surface of bilateral temporal lobes with a standard deviation of 2.2 mm in the superior-inferior direction. In 16 right handed males, the right hemispheric N100m dipoles were 6 mm anterior to the left hemispheric dipoles. The whole head MEG is suitable to see small but significant differences of bilateral cerebral function, with exceptionally high spatial resolution, confirmed by the MRI-linked system.

Detection of epileptiform activity by human interpreters: blinded comparison between electroencephalography and magnetoencephalography.

Summary:  Purpose: Objectively to evaluate whether independent spike detection by human interpreters is clinically valid in magnetoencephalography (MEG) and to characterize detection differences between MEG and scalp electroencephalography (EEG).

High-Frequency γ-Band Activity in the Basal Temporal Cortex during Picture-Naming and Lexical-Decision Tasks

γ-Band activity (GBA) in electroencephalograms (EEGs) has been shown to reflect various cognitive processes. GBA has typically been recorded in the 30-60 Hz range in scalp EEGs. Recently, task-related “high GBA” (HGBA) with frequencies up to 100 Hz has been observed in studies with invasive electrocorticograms (ECoGs). In the present study, we recorded ECoGs from the bilateral basal temporal cortices in a patient with epilepsy and evaluated the task-related HGBA (most prominently in the 80-120 Hz range) accompanying picture-naming and lexical-decision tasks. We examined picture naming using two categories (line drawings of animals and tools). The lexical-decision task was performed using words and pseudowords of two distinct Japanese writing forms, kanji (morphograms) and kana (syllabograms). Task-related HGBA was observed bilaterally during the naming task. Recordings from some electrodes revealed significant differences in HGBA between animal and tool pictures. In contrast to the naming task, there was apparent left dominance in the lexical-decision task. Furthermore, significant differences in HGBA were observed between the Japanese kanji and kana words and between the kanji words and kanji pseudowords. A number of differences in the HGBA observed in the recordings from the basal temporal area were consistent with previous findings from neuroimaging and patient studies and suggest that HGBA is a good correlate of visual cognitive functions.

Surgical Implications of Neuromagnetic Spike Localization in Temporal Lobe Epilepsy

Summary:  Purpose: To investigate the clinical usefulness of magnetoencephalography (MEG) as a guide to the surgical treatment of temporal lobe epilepsy (TLE).

Accuracy and limitation of functional magnetic resonance imaging for identification of the central sulcus: comparison with magnetoencephalography in patients with brain tumors.

The aim of the present study was to clarify the accuracy and limitation of functional magnetic resonance imaging (fMRI) for the identification of the central sulcus affected by brain tumors. Twelve normal volunteers and 11 patients with intracranial tumors adjacent to the central sulcus underwent fMRI and magnetoencephalography (MEG). Three patients were evaluated again after surgery. fMRI was performed with a 1.5 Tesla scanner during repetitive opening and closing of each hand. Cross-correlation function was used to identify activation areas, and the central sulcus was defined as the nearest sulcus to the highest activation spots that were determined by elevating correlation coefficient threshold. Somatosensory-evoked fields were measured using a whole head MEG system. The central sulcus was defined as the nearest sulcus to the N20m for the median nerve stimulus. fMRI and MEG coincided in defining the central sulcus in all 24 hemispheres of volunteers and all 10 examined nonaffected hemispheres of patients. The fMRI-defined central sulcus coincided with the MEG-defined central sulcus in nine (82%) but did not in two (18%) affected hemispheres of patients. The preoperative mismatch disappeared after surgery in one of the two patients. The present study indicates that fMRI successfully defined the central sulcus in most of the patients with brain tumors. However, in a few cases, fMRI was not reliable probably due to venous flow changes by tumor compression and/or compensational activity by brain tissues surrounding the primary sensorimotor cortex. For precise functional assessment of the brain affected by intracranial tumors, combination of fMRI and MEG will be recommended.

Neuromagnetic evidence that the P100 component of the pattern reversal visual evoked response originates in the bottom of the calcarine fissure

Visual evoked magnetic fields due to pattern reversal stimuli were measured in 5 normal subjects using a helmet-shaped 66 channel magnetoencephalography system linked to MRI. The magnetic topography of the prominent 100 ms response (P100m) evoked by fullfield visual showed a double-dipole pattern in the occipital areas of all subjects. Right or left half-field stimuli and upper or lower quadrant-field stimuli evoked a single-dipole pattern in the contralateral occipital area. The P100m sources were then localized using a current dipole model and superimposed on MRI images of each subject. The visual cortex was morphologically variable among the subjects, but the P100m dipoles were all localized at the lateral bottom of the calcarine fissure. Moreover, these P100m dipoles had similar orientations for both half-or quadrant-field stimuli. These results suggest that the P100m is located in a smaller part of the striate cortex than previously reported.

Results of stereotactic aspiration in 175 cases of putaminal hemorrhage.

Believing that improved therapeutic results in cases of intracerebral hematoma might be obtained by minimal invasion of the brain, we used computed tomographic-guided stereotactic aspiration in 175 of 241 patients with putaminal hemorrhage. These patients, who were treated 6 or more hours after onset, had hematomas larger than 8 ml and were unable to raise an arm and/or leg on the affected side. Craniotomy was performed in 15 other patients, most of whom were brought to the hospital with large hematomas within 6 hours of onset. The remaining patients either had mild deficits of consciousness (33 patients) or severe deficits and/or were elderly (18 patients) and were treated conservatively. Thirteen patients (7.4%) showed rebleeding after stereotactic aspiration (6 instances of major and 7 instances of minor rebleeding). Craniotomy and removal of the hematoma were required in three of these patients. Aspiration should be avoided in patients who have a tendency for bleeding, even if mild, because rebleeding occurred in 6 of 23 such patients (26%) in these study. The consciousness level improved in 66 patients (38%), was unchanged in 103 patients (59%), and was worse in 6 patients (3%) 1 week postoperatively. Motor function of the arm improved in 55 patients (31%) and was worse in 23 patients (14%). Six months after surgery, the results for the 175 patients who underwent stereotactic aspiration were: 19% excellent, 32% good, 35% fair, 7% poor, 6% dead, and 1% unknown. For the entire series of 241 patients, the results were: 24% excellent, 26% good, 31% fair, 7% poor, 11% dead, and 1% unknown.(ABSTRACT TRUNCATED AT 250 WORDS)

Ipsilateral Area 3b Responses to Median Nerve Somatosensory Stimulation

Magnetoencephalography investigation of the somatosensory evoked fields for median nerve stimulation detected ipsilateral area 3b responses in 18 hemispheres of 14 (1 normal subject and 13 patients with brain diseases) among 482 consecutive subjects. The major three peaks in the ipsilateral response were named iP50m, iN75m, and iP100m, based on the current orientation in the posterior, anterior, and posterior directions and the latency of 52.7 +/- 6.2, 74.1 +/- 9.4, and 100.2 +/- 15.8 ms (mean +/- standard deviation), respectively. The moment of the iP50m dipole (9.4 +/- 5.7 nAm) was significantly smaller than that of the N20m dipole of the contralateral response (cN20m, 27.5 +/- 10.5 nAm, P < 0.0001). Dipoles of iP50m and cN20m were similarly localized on the posterior bank of the central sulcus. iP50m in the present study had the same current orientation as and peak latency similar to that of the first ipsilateral primary somatosensory response to lip stimulation in our previous report. Therefore, the somatosensory afferent pathway from the hand may reach directly to the ipsilateral area 3b at least in part of the human population.

Middle and long latency peak sources in auditory evoked magnetic fields for tone bursts in humans

The relative position of the P50m and the N100m sources of the auditory evoked magnetic field remains unclear. Magnetoencephalography was performed in 24 normal subjects. Contralateral P50m to left and right ear stimulus was observed in 21 and 19hemispheres, respectively. Ipsilateral P50m to left and right ear stimulus was observed in 17 and 16hemispheres, respectively. N100m was observed in all subjects for all stimuli. Relative position of the equivalent current dipole of the P50m was 1.0+/-7.6 (mean+/-SD) mm posterior, 2.0+/-5.8mm inferior and 1.8+/-8.0mm medial to the N100m dipole position considering all observations. We suggest that the P50m and N100m sources are colocated in an extended area of the cortex.